In air, metallic conductors are capable of transmitting electrical impulses at speeds approaching the speed of light. However, bare conductors are not used in most applications due to the need to mechanically support the conductors and hazards such as shorting, shocking and fire which can occur when other materials are too close to or directly contact electrically transmitting conductors. To avoid such problems and hazards, conductors have been surrounded by non-conducting insulation. On an atomic level, this insulation contains charges which can be polarized and interact with the electrical impulse traveling along the conductor. This interaction retards the flow of the electrical impulse through the conductor and limits the velocity of propagation of the electrical impulse transmitted.
The dielectric constant, K, of a material is a measure of the polarizability of the charges within the material. As the dielectric constant increases, the velocity of propagation of an electrical impulse traveling along a conductor surrounded by the higher K material decreases. Conversely, as the dielectric constant decreases towards 1.0, the dielectric constant of air, the velocity of propagation of an electrical impulse traveling along a conductor surrounded by the lower K material approaches the speed of light. Conventional insulation has a dielectric constant significantly greater than that of air. Consequently, conductors within such conventional insulation transmit electrical impulses at speeds lower than those of a bare conductor in air.
Recent advances in the capabilities of computers and telecommunications demand higher velocities of propagation over insulated conductors. Various insulating materials and configurations have been disclosed but there is still a need for an insulated conductor that can transmit electrical signals at speeds close to the speed of light.
Yoshimura et al., U.S. Pat. Nos. 3,309,458 and 3,332,814, and Japanese patent publication JP 73/5355 disclose foamed polystyrene insulation. In the Yoshimura et al. patents, the foamed polystyrene is hard and brittle, can have up to 97% greater volume than the unfoamed polystyrene and a dielectric constant as low as 1.03. The foamed polystyrene is adhered to flexible polyethylene tape to compensate for the inflexibility of the polystyrene. The flexible tape/foamed polystyrene construct is used as the insulation between the two conductors in a coaxial cable.
Japanese patent publication JP 73/35355 discloses an insulated wire produced by extrusion coating a conductor with an insulative foam composition consisting of polystyrene, a blowing agent and water. The percentage of voids in the foam insulation is 90.6%.
Wilkenloh et al., U.S. Pat. No. 4,107,354, and Japanese patent publication JP 56/167201 disclose foamed polyolefin insulation. Wilkenloh et al. disclose a coaxial cable in which the core conductor is coated with a dielectric having a dielectric constant in the range of 1.32 to 1.1. The dielectric comprises an extruded cellular polyethylene or polypropylene which has been rendered cellular by direct injection of a blowing agent in a liquid form into the polymer during extrusion. The resulting polyolefin has a foam density between 0.10 and 0.21 grams/cubic centimeter (g/cc).
Japanese patent publication JP 56/167201 discloses an insulated wire comprising foamed polyolefin extruded onto a conductor. The foamed polyolefin is a copolymer of ethylene and alpha-olefin having more than four carbon atoms, containing a foaming agent. The starting (unfoamed) copolymer has a density of 0.926 to 0.960 g/cc. The extruded polyolefin has a foamed percentage of 82%.
Suzuki, U.S. Pat. No. 4,379,858 discloses a copolymer resin matrix containing porous fragments dispersed therein. The resin matrix comprises a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer. The porous fragments have a porosity of 40-90% and are selected from the group consisting of polytetrafluoroethylene (PTFE), polyethylene, and polypropylene. Air trapped in the fragments expands during melt molding to form bubbles in the matrix. The product is suitable for insulating wire or cable.
Perelman, U.S. Pat. Nos. 4,304,713 and 4,368,350, and British patent publication GB 2,143,237 disclose foamed fluoropolymers as electrical insulation. Perelman discloses a foamed melt extrudable perfluorinated ethylene-propylene polymer containing a PTFE nucleating agent. Preferably, the foamed perfluoropolymer resin has a closed cellular structure with cells ranging in size from 10 to 40 mils (0.25-1.02 mm), a foamed density as low as 1.0 g/cc, and an insulation loss of less than 1.8 db/100 ft. at 1000 MHz. The foamed resin can be used to make jacketed electrical conductors and/or coaxial cables wherein at least one conductor is bonded to the foamed melt extrudable resin.
British patent publication GB 2,143,237 discloses foamed fluoropolymer insulation which is melt-extrudable onto a wire in a process for making lightweight coaxial cable. Preferred foamed insulation has a reported void level of 65%.
European patent publication EP 211,505 and Gore, U.S. Pat. Nos. 3,953,566 and 4,096,227, disclose porous PTFE insulation. EP 211,505 discloses a flexible, high voltage electrically insulating tape comprising a tape of expanded porous PTFE having a fluoroelastomer impregnated within the pores thereof at at least one surface of the PTFE tape. The impregnated porous PTFE has a dielectric constant as low as 1.3. The fluoroelastomer is filled in the surface pores to prevent the internal open-cells from being crushed by external forces. To insulate a wire or cable, the modified PTFE tape is spirally wound on or longitudinally attached to the conductor.
In U.S. Pat. Nos. 3,953,566 and 4,096,227, Gore discloses an expanded, amorphous-locked PTFE useful as an electrical insulator. The PTFE has an amorphous content exceeding about 5% and a microstructure characterized by nodes interconnected by fibrils (i.e. open cells). The expanded PTFE insulation in a coaxial cable has a dielectric constant of 1.2 to 1.8.